1. Field of the Invention
The present invention relates to an optical receiver which performs data communication using an optical signal.
2. Description of the Background Art
As conventional optical transmission systems, there are optical fiber transmission in which an optical signal is transmitted through an optical fiber, optical wireless transmission in which an optical signal is transmitted through free space without using an optical fiber, and the like. Concerning an optical receiver for use in these optical transmission systems, it is a challenge to improve the efficiency of a photodetector (PD) which receives an optical signal from a transmitter. Particularly in the case of optical wireless transmission, it is necessary to couple an optical signal output from an optical transmitter with the photodetector of an optical receiver by accurately aligning optical axes of the optical transmitter and the optical receiver and suppressing spread of an optical beam using a lens or the like.
A method for optical axis alignment has been proposed in Japanese Patent Laid-Open Publication No. H05-183513, in which an optical signal for optical axis alignment is transmitted apart from an optical signal containing a data signal. Optical axis alignment is performed in advance, and after the optical axis alignment the optical signal containing a data signal is transmitted so that optical wireless transmission is achieved.
In the above-described conventional method in which optical axis alignment is performed in advance, it is necessary to narrow the diameter of an optical beam emitted from an optical transmitter in order to improve the coupling efficiency between an optical signal and a photodetector. However, it is technically difficult to narrow the optical beam diameter to a level which effectively enhances the coupling efficiency. In addition, even when an optical beam is narrowed under a certain positional relationship, the coupling efficiency is reduced if the positional relationship is lost. Therefore, the method of narrowing the optical beam diameter is not very practical.
Therefore, when the optical beam diameter is large, the coupling efficiency cannot be improved unless a photodetector 100 has a large light receiving diameter as illustrated in (b) of FIG. 10. However, as the light receiving diameter of the photodetector 100 is increased, the capacitance of the photodetector 100 increases. As a result, the frequency response characteristics are deteriorated, so that the transmission rate of a data signal is limited (the transmission rate is slowed).
In order to take measures against the case where the optical beam diameter is large, a plurality of photodetectors having a small light receiving diameter may be used, and signals obtained by the plurality of photodetectors are preamplified, and thereafter added. However, in such an analog technique, the sum signal after preamplification has a large amplitude, and a subsequent circuit needs to have a considerably large dynamic range. Also, since the sum signal has a large amplitude, the optical receiver has large power consumption.